1. Field of the Invention
This invention relates to a technique for compensating a modulated laser with respect to temperature variations.
2. Description of the Prior Art
High speed lightwave communication systems typically comprise a semiconductor laser that is modulated by a digital electrical signal. One important characteristic of the laser is the light output verses electrical drive current amplitude. It is known that this characteristic changes with respect to aging and temperature. That is, the threshold current, which is the current at which the lasing commences, generally increases as the laser ages, and as the temperature increases. This phenomenon is referred to as "threshold shift", and is illustrated in FIG. 10. The laser light output power (L) verses current through the laser (I) is shown for two temperatures, where T2&gt;T1. It can be seen that the threshold is greater at the higher temperature; that is, Ith2&gt;Ith1. Furthermore, the slope of the curve above threshold tends to decrease as the temperature of the laser increases. That is, the light output for a given current above threshold is less at the higher temperature. The ambient temperature variations may be quite large, and the laser and its drive circuitry may produce significant heat output. Therefore, the change in light output due to temperature changes is a significant consideration in the design of laser transmitter systems.
One known solution to the problem of temperature changes is to maintain the laser temperature relatively constant during operation. This stabilization may be accomplished by mounting the laser on a thermoelectric cooler (TEC). Within a certain ambient temperature range, this technique maintains the laser junction temperature constant, and provides thermal isolation between the laser and the driver circuitry, so that ambient temperature variations do not significantly increase the laser temperature. In this manner, the electrical drive current may be maintained at a relatively constant level during operation, while still obtaining constant light output. In addition, the use of a cooler helps to increase the longevity of the laser, and also helps maintain the wavelength of the laser at a constant value. However, these considerations are relatively less significant in some systems, especially with the increases in laser lifetimes that are being achieved with newer laser designs.
The electrical drive current for the laser is typically formed from two components, being a bias current and a modulation current having a constant amplitude. Referring to FIG. 10, the bias current (Ib1) is typically adjusted so as to maintain the laser just below threshold (i.e., off) in the absence of modulation current, at room temperature (T1). When the modulation current (Imod1) is on, the total drive current is then above threshold, so that the laser produces light output. In order to adjust the bias current to compensate for threshold shift, a "backface monitor" is typically used. This includes a photodetector mounted so as to receive a portion of the light output emitted from the back face of the laser diode. The electrical output of the photodetector is then used to control the bias current. Therefore, as the lasing threshold increases, as due to temperature increases or other factors, the bias current is increased so at to maintain total light output constant.
However, the use of backface monitoring does not solve the problem of laser output changes as a function of temperature, because that technique maintains only the total laser light output constant (e.g., at Lset). That is, in prior art designs, if the bias current is adjusted to be just below threshold at low temperatures, then the backface monitor tends to shift the bias current too high at high temperatures (Ib2), so that the laser does not turn fully off when no modulation signal is present. Therefore, the above-noted cooling technique has frequently been adopted, especially in high performance systems, to negate temperature effects on performance.